It is certainly a bit of a misnomer to use the term "underpaid" when discussing professional baseball players. With a new league minimum salary of $480,000, even the sixth outfielder and the lefty specialist make more annually than the President of the United States. Make no mistake—if you're talented enough to catch the eye of a big league team, you have the chance to become better paid than the majority of Americans. The league minimum is nothing to sneeze at, but the league average is a tick above $3 million—a paycheck most of us would be hard-pressed to complain about.
Of course, all players—and all player contracts—are not created equal. They are frequently based on past performance and hope to approximate future performance, an inexact science at best. It is not uncommon for aging players to perform significantly worse in the final years of their contracts than they did when they first signed it, and every now and then you get players whose numbers take drastic and unexplained dips soon after signing mega-deals.
These so-called "albatross contracts" haunt the dreams of every major league GM, who want nothing less than to sign the next Dan Uggla ($13 million, batting .210), Bobby Abreu ($9 million, recently designated for assignment by the Dodgers) or Vernon Wells ($21 million for a whopping -.4 WAR per Fangraphs).
But on the other end of the spectrum are players who are playing well above their pay grade. These are mainly younger players who have yet to become arbitration eligible, but have already started to contribute in big ways to their big league club. For some of these players, the right break for their team in the playoff race could mean a big boost in their MVP candidacy (as we have seen in recent years, MVP voters very much factor team success into the spirit of the award).
We can quantify the extent to which a player outperforms their paycheck by calculating their cost vs. performance score—the average MLB salary divided by the player's salary, multiplied by the player's WAR.